Seeing as there already is a well founded explanation of gravity, why would Hawking have his own? And no, its not a change in gravity that affects the average size of creatures on Earth, but rather the amount of biomass in the ecosystem that is available.

Not necessarily. Plants as well as animals evolve independent of each other.

"In analyzing our interactome map of Arabidopsis, we found strong evidence of this rapid-then-slow process over the hundreds of millions of years of the plant's evolution," says Braun. "It provides the first-ever empirical evidence of evolution acting on protein networks."